Currently, the most widely used process for welding aluminum and its alloys is the TIG (Tungsten Inert Gas) process which makes it possible to obtain weld beads whose quality and compactness are superior to those obtained using MIG (Metal Inert Gas) processes.
However, the TIG process cannot be regarded as being completely satisfactory as it results in welding speeds which are markedly less than those of MIG processes and therefore results in lower productivity.
In fact, the compactness problem (porosity of the weld beads) stems from hydrogen (H.sub.2) having a high solubility in the molten metal.
In other words, the formation of pores by the incorporation of hydrogen is greater the higher the temperature of the material is above its melting point. By way of example, the curve of the solubility of hydrogen in pure aluminum as a function of the temperature of the metal is given in FIG. 1.
In welding, there are conventionally several sources of hydrogen, generally contaminants, such as hydrocarbons (greases, oils, etc.) or water vapour, present on the workpieces to be welded and/or on the wire, the moisture present in the pipes and the gas bottle or bottles, the air inlets or leaks in the fittings connected to the gas pipes and internal condensation occurring in the welding nozzle.
During the welding operation, these various contaminants dissociate in the electric arc, giving, in particular, hydrogen which dissolves immediately in the pool of molten metal and the droplets in the welding arc.
Next, the convection movements in the pool of molten metal transport the liquid which is saturated with hydrogen into the cooler regions of molten metal, thus resulting in the formation of pores during the process of solidification of the metal by cooling.
The incorporation or loading of hydrogen is, according to the Sievert law, proportional to the square root of the hydrogen partial pressure above the pool of molten metal.
In order to minimize this problem, the following are generally carried out:
a pretreatment of the workpieces to be welded, such as degreasing, brushing or scraping, followed by subsequent storage for a short time of the workpieces protected from the aforementioned contaminants until they are actually welded; PA1 storage and use of the filler metal (wire) in a sealed, inerted and heated pay-out; PA1 choice of electrical parameters making it possible to avoid turbulence in the gaseous shielding with wet-air ingress, in particular those for maintaining a short and controlled arc length; PA1 choice of welding position conductive to degassing; and/or PA1 choice of welding speed suitable for favorable removal of the dissolved gases (degassing of the liquid metal). PA1 does not have the above-mentioned drawbacks; PA1 makes it possible to obtain effective degassing of at least most of the diffusible hydrogen liable to be in the pool of molten metal, and thus to improve the appearance and quality of the weld beads considerably; PA1 results in effective welding of workpieces, especially those made of aluminum and its alloys; PA1 is easy to employ and is less expensive than the conventional processes on an industrial scale; and PA1 can be used in both manual welding and automatic welding. PA1 the modulation background time is, depending on the wire speed, between 2 ms and 20 ms, preferably between 4 ms and 15 ms (ms standing for millisecond), and more preferably in the range 5 ms to 12 ms; PA1 the current pulse has a waveform chosen from square, sinusoidal, triangular, trapezoidal and rectangular waveforms and combinations thereof, preferably a square or rectangular waveform; PA1 the current pulse is applied to a wire whose diameter is at least 0.8 mm and preferably from approximately 1 mm to approximately 1.6 mm; PA1 the welding speed is at least 1 cm/min. and preferably less than 5 m/min., and preferably in the range 20 cm/min. to 1 m/min.; PA1 it furthermore comprises a regulation of the arc length, based on a reference measurement obtained from the peak time and/or the background time by varying the peak current and/or the background current, the arc length preferably being between 5 and 30 mm, preferably between approximately 10 and 20 mm; PA1 it furthermore comprises maintaining a minimum current difference between the peak current and the background current of at least 30 A and preferably at least approximately 100 A, preferably from 130 to 250 A; PA1 it is carried out under a flow of a shielding gas chosen from helium, argon and mixtures thereof, the gas optionally furthermore containing carbon dioxide (CO.sub.2) and/or oxygen (O.sub.2) in small amounts, i.e. in a proportion of less than 2%, preferably less than 1.6% or even less than 1%; PA1 a meltable welding wire is melted so as to transfer droplets of molten metal in spray mode, the speed of the welding wire being from 1 to 20 m/min., preferably from 3 to 13 m/min.; PA1 the intensity of the background current (Ib) is between 20 and 60 A, preferably approximately 30 A; PA1 the intensity of the high current or peak current (I.sub.p) is between 150 A and 350 A, preferably between approximately 190 A and 310 A; PA1 the mean intensity (I.sub.m) of the current is between 100 A and 300 A, preferably between approximately 105 A and 280 A; and PA1 the rate of detachment of the metal droplets from the melting of the meltable wire is between 200 and 700 droplets/s, the metal being transferred only when the current is at its peak. PA1 means for adjusting the current pulse, making it possible to obtain a current pulse of a defined waveform chosen from square, sinusoidal, triangular trapezoidal and rectangular waveforms and combinations thereof; PA1 means for controlling the arc length, making it possible to control the arc length based on a reference measurement obtained from the peak time and/or the background time varying the peak current and/or the background current; and/or PA1 current control means, making it possible to maintain a minimum current difference between the peak current and the background current of at least 30 A and preferably at least approximately 100 A. PA1 gas shielding is used which contains at least 90% (by volume) of at least one major gaseous component and at most 1.95% (by volume) of at least one minor gaseous component, preferably from 0.01% to 1.80% of the at least one minor gaseous component, in particular from approximately 0.5% to 1.6% of oxygen; and PA1 the current is modulated at a modulation frequency of less than 60 Hz with, preferably, a modulation background time of between 2 ms and 20 ms. Preferably, the modulation frequency is in the range from 10 to 50 Hz, preferably in the range of approximately 15 to 39.5 Hz and advantageously is at least 30 Hz.
However, it will be readily understood that these precautions are very restrictive and, in some cases, result in an appreciable increase in the overall cost of the welding process.
Another technique consists of using a filler metal containing microalloys, such as cobalt, which are intended to increase the number of nucleation sites, thus reducing the size of the pores contained in the weld bead. However, this technique does not reduce the overall porosity of the bead and as a result its quality is improved only slightly, if at all.
Furthermore, another known method is agitation of the welding pool, either by direct mechanical agitation, such as by vibration, or by applying electromagnetic fields external to the welding circuit. Thus, a direct-current TIG welding process, with a negative-polarity electrode and with or without a filler metal, has already been described which employs electromagnetic agitation by applying an alternating external magnetic field perpendicular to the workpieces to be welded to each other. However, the results obtained show that, at a frequency of 15 Hz, the agitation causes higher porosity than without an electromagnetic field.
Moreover, document U.S. Pat. No. 3,409,756 teaches a spray-type arc welding process employing periodic variations of the arc power between a high value and a low value, for example between 18,600 and 12,500 watts.
On the other hand, document U.S. Pat. No. 3,956,610 discloses a process for welding ferrous metals, steels and non-ferrous alloys employing periodic variations of the electric current delivered to the electrode.
Document EP-A-0,607,819 describes a pulsed-arc MAG (Metal Active Gas) welding process with an oxygen gas flow for welding galvanized materials, such as zinc. This document therefore seeks to solve a technical problem different to that of the present invention, namely that of avoiding contamination of the weld by zinc vapour forming at the time of welding.
Furthermore, a pulsed MIG or TIG welding process has also been described which employs large variations in the welding current so as to obtain a refinement with a different orientation of the structure of the metal deposited.
Another document U.S. Pat. No. 5,508,493, describes an MAG welding device making it possible to improve the appearance of the weld beads.
Document U.S. Pat. No. 4,273,988 relates to a pulsed-arc welding process with a shielding gas consisting of 60% helium, 25% argon and 15% carbon dioxide and with metal transfer in the spray mode at a frequency of 90 to 400 Hz.
Similarly, document U.S. Pat. No. 4,507,543 describes a pulsed-arc plasma or TIG welding process and document U.S. Pat. No. 4,749,841 relates to a pulsed-arc welding process with a shielding gas consisting of 16-25% helium, 1-4% carbon dioxide and the balance being argon.
Furthermore, document EP-A-422,763 describes a pulsed MIG welding process, especially for motor-vehicle parts, with a shielding gas consisting of argon containing from 2 to 5% oxygen.
However, none of these known processes makes it possible to solve the problem posed, namely to obtain really effective degassing of the gaseous impurities liable to contaminate the pool of molten metal, in particular diffusible hydrogen, and therefore does not result in joints or weld beads whose appearance and quality are improved and compatible with stringent industrial requirements.